@SUNRAY: wake up and smell the coffee! Our sawmilling friend BDFT says he has an "early 1900s sporting rifle", so I am doing him the courtesy of assuming that he knows what he owns.
SOME Lee-Enfields were slapped together out of spares bins in the USA in the 1960s... but certainly NOT, back at ANY time before that. The vast majority of LE rifles ARE ex-military, and that includes my 1896 Sparkbrook (sportered in the early 1920s) and my A.G. Parker custom job, built originally in 1894, rebuilt in 1920; there is NOTHING wrong with them, nor with the manner in which they were assembled. According to YOU, almost a dozen Factories, BSA, Parker-Hale, George Gibbs, Holland and Holland, A.G. Parker, the Fultons, Alex Martin and several thousand Armourers didn't know what they were doing: give your head a shake! Remember, THESE were the last people to work on these rifles when they were Sold Out of Service, before they got to the local hardware store.
An old Machine-Gunner (Yukon Machine Gun Company, 1914 and Canadian Motor Machine Gun Brigade, 1918) once told me that, "There are 48 possible stoppages in the Lewis Light Machine Gun..... and 47 of them are in the Magazine!". Being that he LIVED long enough to tell me that, I would assume that he knew what he was talking about. And he was certainly RIGHT: ammunition problems account for nearly ALL "rifle problems". There is a great deal MORE time and attention expended in the manufacture of a thousand-dollar Machine Gun than there is in the making of a SEVEN-CENT Cartridge. Today, there is NO commercial ammunition made which duplicates precisely the Military Specifications for the.303 Ball Cartridge..... or even the CASING, for that matter.
Yes, bore and groove diameters DID vary during Wartime but nearly all of the really bad ones were changed-out during the massive FTR programs following the Great War, the Second World War and the Korean War. A combination of FLAT-based bullet and a quick powder brings MOST of the survivors under control...... as you have seem me post literally a HUNDRED times over the past eight years.
MANY ammunition problems and SOME Rifle problems may be bypassed for the cost of a simple Pony-tail Tie (one-fifth of a cent at your local Dollar Store, plus tax: not really excessive). Generally, this includes the numbing, terrifying, esoteric issue of HEADSPACE. Following the use of the Pony-tail tie on a Cartridge, you simply NECK-SIZE your brass, which now headspaces on the SHOULDER rather than on the often-defective RIM of the round.
@ BDFT, the Original Poster: Friend, I happen to like early .303 Sporters almost as much as I like the rifles from which many of them were made. Would you be so kind as to give us a nice photo or two of your rifle, one full-length and another of what markings might be on the Butt Socket..... and one of the Knox-form? Thank you. We'll get this sorted somehow.